The present invention relates to a desalinating drip-irrigation system for producing low cost irrigation water for agriculture starting from brakish well water or sea water. More particularly, the invention is directed to a system for use adjacent a plant supporting material to be irrigated, for converting salt water into fresh water dispensing the fresh water thus produced dripwise to the plant supporting material.
Desalination or the conversion of salt water into fresh water has been the subject of extensive research for many years. In particular, solar desalination has been of great interest in this age of high energy costs. Solar desalination is however characterized by low yields of fresh water and normally requires large surface areas and significant capital expense. However, operating costs are minimized since the energy of the sun is used as heating energy and is thus free, and only moderate maintenance is required.
Agricultural development in the arid areas where solar desalination has been of interest has benefited tremendously over the last few years from development of a technique called drip-irrigation. Drip-irrigation is characterized by huge decreases in water consumption for a given crop with actual increases in productivity for other biological reasons. As the name suggests, irrigation is performed using drips at ground level rather than by overhead spraying. This minimizes evaporation and run-off losses and results in a small but adequate amount of water percolating down to the roots of the plants to be irrigated. The percolation does not occlude air and hence results in improved plant performance. Typical water quantities used are on the order of 2 liters per meter of crops per day; however, overhead spraying may use 100 times this amount of water and provides lower plant performance.
In a typical solar still installation, salt water contained in a black trough covered with a sloping glass roof is heated by the sun and vaporizes slowly. The water vapor reaches the cooler glass surface by convection where it condenses, forming a film of distilled water which runs off into a collecting trough and from there to storage. Another form of solar still which is described in U.S. Pat. No. 3,785,931 comprises a vertical microporous evaporator tube housed within, but separated from, a transparent condenser tube. Salt water absorbed by the porous evaporator is heated by sunlight passing through the transparent condenser to produce water vapor which subsequently condenses on the inner surface of the condenser. As condensed water collects on the condenser tube, it runs down to be collected as purified water. The condensate, however, is not used for irrigation purposes.
U.S. Pat. No. 2,807,912, on the other hand, discloses a pelagic solar still for producing fresh water from sea water and for irrigating plants on the surface of the ocean with the fresh water thus produced. This pelagic solar still comprises a receptacle adapted to float on the surface of the ocean and containing at the bottom thereof a bed of plant supporting material such as sand or soil. The receptable is covered with a transparent glass or plastic sheet, beneath which is a wick extending over the receptable and down into the sea water, the portion of the wick underlying the transparent sheet being on the interior of the receptable. Sea water carried by the wick to the area immediately below the transparent sheet is heated by solar radiation passing through the sheet, and the resulting water vapor condenses on the underside of an inwardly and upwardly extending portion of the receptable. The condensate runs down to the bottom of the receptable and collects as fresh water which serves to irrigate plants. Such a solar still is thus essentially limited to supporting plant growth on the surface of the ocean and cannot of course be used on land for drip-irrigation.